This invention relates to a pneumatic linear actuator, and more particularly to a digitally controllable pneumatic linear actuator.
Linear actuators, such as hydraulic cylinders, have hitherto been widely employed as driving units for feeding slides or tables of machine tools and linearly moving parts of other machines. However, such linear actuator has various demerits as follows:
(I) The actuator of this type cannot be directly controlled by a digital controller such as a computer.
(II) The actuator of the above type cannot feed a moving part as precisely as a known actuator of another type comprising an electric motor and a feed screw does.
(III) It is difficult for the linear actuator to precisely position a moving part without means such as a positive stopper, a dog or a limit switch disposed in the traveling path of the moving part.
While the actuator of the type above described has such demerits, it has various merits over the known actuator comprising the electric motor and the feed screw in that it can impart a quicker motion to a moving part and is less expensive and smaller in size than the latter type of actuator. It has therefore been strongly demanded to develop a linear actuator which can be directly controlled by a digital controller such as a computer and which eliminates the necessity for providing a stopper, a dog or like position detecting element in the traveling path of a moving part.
A hydraulic linear actuator satisfying the above demand has been recently developed and put into practical use. The linear actuator of this type can be directly controlled by a digital controller such as a computer and is provided with an electro-hydraulic servo valve. This known hydraulic linear actuator is, however, defective in that it cannot be used in an environment in which pollution with oil is strictly inhibited by the law or in an environment in which an inflammable fluid or substance exists or predominates. Further, due to the fact that the result of detection of displacement of the piston rod connected to the piston in the hydraulic linear actuator is directly fed back to the digital controller, this known hydraulic linear actuator has such another defect that detection errors tend to accumulate after a plurality of times of use resulting in degradation of the precision of positioning the piston.
It has therefore been strongly demanded to develop a pneumatic linear actuator which can be operated without the defects of the known hydraulic linear actuator. Generally, a pneumatically operated actuator such as an air cylinder or the like has such a basic defect that its piston rod cannot be precisely stopped at a predetermined stop position due to the compressibility of air which is the operating medium. A pneumatic linear actuator of hybrid type provided with a pneumato-hydraulic transducer has been proposed in an effort to obviate this defect of the pneumatic actuators. However, the linear actuator of the proposed type cannot also be used in an environment in which pollution with oil is inhibited by the law, like the hydraulic linear actuator.
An attempt is being made to mechanically brake the piston rod of a pneumatic actuators for the purpose of precisely stopping the piston at a predetermined stop position. In such a pneumatic actuators, mechanical wear of the piston rod is the problem which has to be solved.